The present invention generally relates to a process for controlling the degree of branch of high 1,4-cis polybutadiene, and more particularly to a process of easily controlling the degree of branch of polybutadiene which is concerned directly with physical properties of polymer such as processability or strength, in which process the high 1,4-cis polybutadiene is prepared via polymerization of 1,3-butadiene in the presence of the Ziegler-Natta catalyst by adding a dialkylzinc compound as an agent for controlling the degree of branch, and controlling the degree of branch of polybutadiene based on the content of the dialkylzinc compound without any alternation in the 1,4-cis content or polymerization yield.
According to the conventional method of controlling the degree of branch of high 1,4-cis polybutadiene (hereinafter, referred to as xe2x80x9chigh-cis BRxe2x80x9d), European Patent. No. 093,075 discloses a method of using organo-nickel compound, organo-aluminum compound and hydrogen fluoride as a main catalyst and controlling the degree of branch of high-cis BR based on the content of organo-nickel compound and the variation of the polymerization temperature.
U.S. Pat. No. 3,528,957 and U.S. Pat. No. 3,483,177 also disclose a method of using organo-nickel compound, organo-aluminum compound and boron trifluoride (BF3) compound as a main catalyst and controlling the solution viscosity of high-cis BR based on the type and content of the catalyst, thus improving processability and physical properties of polymer. This method is based on the fact that the degree of branch decreases with an increase in the solution viscosity at a constant Mooney viscosity.
Furthermore, U.S. Pat. No. 3,464,965 discloses a method for controlling the cold flow tendency of high-cis BR which is closely concerned with processability and workability. This method is based on the fact that the cold flow tendency of the high-cis BR decreases with an increase in the degree of branch. More specifically, the method involves adding organo-nickel compound, organo-aluminum compound and boron trifluoride (BF3) compound as a polymerization catalyst for polybutadiene and varying the content of the diene compound added to the organo-nickel compound prior to the polymerization reaction to control the degree of branch of the high-cis BR.
The solution viscosity and the Mooney viscosity of high-cis BR are greatly dependent upon the molecular weight, i.e., they increase with an increase in the molecular weight of high-cis BR. Many methods of controlling the molecular weight to improve processability and physical properties of the high-cis BR have been suggested. For example, U.S. Pat. No. 5,100,982 discloses a method of using organo-nickel compound, organo-aluminum compound and boron trifluoride (BF3) compound as a main catalyst, together with halogen-substituted phenol derivative as an additive to control the molecular weight and the molecular weight distribution of the high-cis BR.
U.S. Pat. No. 5,451,646 also discloses a method of using organo-nickel compound, organo-aluminum compound and fluorine-containing compound as a main catalyst, together with p-styrenated diphenyl amine to control the molecular weight of the high-cis BR, thereby improving the processability.
Further, Japanese Application No. 51-127030 discloses a method of preparing the high-cis BR with a narrow range of molecular weight distribution using nickel compound, boron compound, alkyllithium and alkylbenzene sulfonate.
In addition, U.S. Pat. No. 4,533,711 discloses a method of further extending the molecular weight distribution, wherein rare earth metal compound belonging to the atomic number of 57 to 71, organo-aluminum compound and halogenated aluminum compound were employed as a main catalyst, while using organo-aluminum hydride or hydrocarbon compound containing activated hydrogen as a controller of molecular weight.
However, the conventional methods of controlling the degree of branch and the molecular weight distribution in preparing the high-cis BR are problematic in that the polymerization yield and the 1,4-cis content are lowered with great complexity in the process for industrial production.
Typically, the degree of branch together with the average molecular weight and the molecular weight distribution is directly concerned with the processability and physical properties of polymer, which depends on the ratio of solution viscosity to Mooney viscosity. That is, the linearity of polymer increases with reduced degree of branch as the ratio of solution viscosity to Mooney viscosity increases.
Generally, a polymer of a low degree of branch, i.e., high linearity has an increase in the cold flow tendency and thereby results in a deterioration of processability and working performance in carriage and storage. Otherwise, a polymer of a high degree of branch is improved in working performance due to reduced cold flow tendency but deteriorated in physical properties.
Considering the above-mentioned problems, there is a need for rubbers having a low molecular weight and a considerably high degree of branch in the manufacture of tires with improved processability, and rubbers having a high molecular weight and a low degree of branch in the manufacture of tires with excellent physical properties such as high impact resistance and high tensile strength.